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Re: [std-proposals] Copy-construct, move-construct, and PR-construct

From: Sebastian Wittmeier <wittmeier_at_[hidden]>
Date: Wed, 13 Sep 2023 18:04:00 +0200
Hi Arthur, so let us focus on the factory function. The value semantics code has to produce the pointer semantics code somehow out of value syntax.   I think it is useful to explicitly mark the returned variable at declaration, as it is located in the memory of the caller. Even if the compiler can deduce it from the returned expression. This is better than marking it (only) at the return IMHO.   std::mutex lockedMutexFactory() {     std::mutex return m; // mark the variable with return keyword;                          // it is not constructed on the stack,                          // but at the location provided by the caller     m.lock();     return m;               // Either: only m allowed here, or,     // return std::mutex(); // alternatively, the destructor of m is called first,                             // and then std::mutex() is returned } The variable marked with 'return' should be of the same type as the function return type. There can be only one variable marked as 'return' (but they can have different names in parallel code blocks, e.g. in the if and the else block). Each path out of the function (return, throw, ...) either uses the existing marked variable, destructs it and/or creates a new object at that caller-provided location or does nothing.   Best, Sebastian   -----Ursprüngliche Nachricht----- Von:Arthur O‘Dwyer via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Mi 13.09.2023 17:19 Betreff:Re: [std-proposals] Copy-construct, move-construct, and PR-construct An:std-proposals_at_[hidden]; CC:Arthur O‘Dwyer <arthur.j.odwyer_at_[hidden]>; On Tue, Sep 12, 2023 at 2:16 PM Sebastian Wittmeier via Std-Proposals <std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]> > wrote: Should we have unmovable-and-uncopiable return values at all?   Even if it can be technically solved for certain cases, those classes are not meant to be moved, copied or returned.  s/or returned// I'd say that such types were always meant to be supported (because why wouldn't they be?), and since C++17 immovable return types have been supported, at least in the cases that Just Work (like `return std::mutex();`). One certainly can't say that std::mutex wasn't "meant" to be returned. It's a class type; of course you can return it from a function! (In spirit, although not literally, each constructor of std::mutex is such a function.)  Better change the interface of std::optional::emplace and/or SomeFunctionThatReturnsMutex(). So that  - SomeFunctionThatReturnsMutex gets a pointer, where to construct its mutex and  - emplace gets a Callable, which creates the mutex at the right moment in time  The problem with that kind of "two-stage initialization" is that it doesn't lend itself to value semantics. You're suggesting that I should write     std::mutex *lockedMutexFactory(void *where) { auto *p = ::new (where) std::mutex(); p->lock(); return p; }     alignas(std::mutex) char buf[sizeof(std::mutex)];     std::mutex *m = lockedMutexFactory(buf);     use(*m);     m->~mutex(); but what I really want to write is     std::mutex lockedMutexFactory() { std::mutex m; m.lock(); return /*somehow*/ m; }     use(lockedMutexFactory());  Now, I don't think any of the half-proposed ideas in this thread so far have been good. But I disagree with the notion that we should somehow desire two-stage initialization just because one of our types happens to be immovable.  FVG wrote:    optional<mutex> om;    om.emplace( SomeFunctionThatReturnsMutex() ); Currently we can't do this, and it's something that needs to change. We can do this fine; use the SCSE pattern. The things we currently can't do are: - Implement SomeFunctionThatReturnsMutex() in the first place, unless it's happy with URVO (i.e. just calls a ctor, or another factory, and does nothing else before returning) - Do this dance for a hypothetical immovable type that is convertible-from any T  my $.02, Arthur -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals

Received on 2023-09-13 16:04:02